1. Technical Field
The present invention relates to a direct current (DC)-DC converter and, more particularly, to a synchronous DC-DC buck converter.
2. Description of the Related Art
A DC-DC converter is a circuit that converts a DC voltage at one level into a DC voltage at another level. A DC-DC converter that converts an input voltage into an output voltage at a lower level and outputs the resulting voltage is referred to as a step-down DC-DC converter. An inductor-type buck converter also called a buck converter or a DC-DC buck converter is representative of the step-down DC-DC converter.
A synchronous DC-DC buck converter includes an inductor, two switches configured to operate in a complementary manner in order to control energy supply from input voltage and energy transfer to output voltage with respect to the inductor, and a capacitor configured to maintain a stepped-down voltage.
Theoretically, a synchronous DC-DC buck converter is a circuit in which a step-down ratio is determined based on the duty ratio of the one of two switches that applies an input voltage to an inductor, and thus is a DC-DC converter that can adjust the level of an output voltage using a simple structure.
However, since it is difficult to implement a high-capacity inductor and capacitor using integrated circuits, the inductor is implemented using an external element and only the remaining switching circuits are implemented in integrated circuits even when integrated circuits are used for implementation.
The size of an inductor is proportional to duty ratio and the difference between an input voltage and an output voltage, and is inversely proportional to switching frequency and the variation rate of the ripple current of the inductor upon switching.
Furthermore, the size of a capacitor is proportional to the variation rate of the ripple current of the inductor upon switching, and is inversely proportional to switching frequency and the magnitude of output voltage.
Accordingly, in order to reduce the sizes of an inductor and a capacitor and implement them in integrated circuits, switching frequency should be increased.
When switching frequency is increased, new problems occur in transistors implemented in integrated circuits in the form of switches. When the two switches of a synchronous DC-DC buck converter are simultaneously turned on, current may flow, or shoot-through, from an input voltage terminal through the switches to a ground terminal. Accordingly, it is necessary to lower the possibility that transistors operating as switches are simultaneously turned on at any time by applying predetermined dead time to head and tail of the complementary conductive time of each of the switches.
Another problem that occurs when switching frequency is increased is electromagnetic interference (EMI).
Since a switching signal is generated to have very stably maintained switching frequency, electromagnetic energy is concentrated on the switching frequency. Even when the voltage level of a switching signal is sufficiently low, a stable and high-frequency switching signal may cause noticeable electromagnetic interference within switching frequency band. Such electromagnetic interference sometimes affects the operations of other electronic devices, and even sometimes deteriorates the performance of a circuit itself.
In general, paradoxically, the possibility of the occurrence of electromagnetic interference based on a stable switching signal can be addressed by making the switching signal unstable. According to this technique, electromagnetic energy is spread over a wide band in which the switching signal is unstably generated, and thus a peak is lowered, with the result that the possibility of the occurrence of electromagnetic interference is eliminated. Such techniques are collectively referred to as “spread spectrum clock generation (SSCG).
However, it is difficult to apply a spread spectrum clock generation technique for eliminating electromagnetic interference to a synchronous DC-DC buck converter.
For example, dead time that is applied to head and tail of switching signal pulses may be problematic. Dead time should be chosen taking into account the turn-on time and turn-off time of a transistor that is used as a switch. For example, switching frequency can be increased by 10%, with the switching period shortened by 10%, according to application of spread spectrum clock, and then fixed dead time may be applied to, resulting that on-duty time of a switch that connects input power to the inductor shortens, shorter than the case in which switching frequency is not spread at all, and output voltage thus decreases.
In contrast, switching frequency can be decreased by 10% with a switching period increased by 10%, and then fixed dead time may be applied to, resulting that on-duty time of a switch that connects input power to the inductor lengthens, longer than the case where switching frequency is not spread at all, and output voltage thus increases.
In such a case, one can expect a problem that the output voltage of the DC-DC voltage converter is not stable and fluctuates with a period corresponding to the spread period of a spread spectrum clock.